This application claims the benefit of European Application No. 00402450.1, filed Sep. 6, 2000 now E.P. 1186918.
1. Field of the Invention
The present invention relates generally to opto-electronic devices, and particularly to an optical mode transformer.
2. Technical Background
Optical mode transformers (OMTs) are known to reduce optical coupling loss by mode transforming between a smaller mode of an optical device and a larger mode of an optical fiber. Spot-size transformation is needed from the 1-2 um range of the (usually elliptical) guided mode of the optical device (such as a laser, for example) to the 8-10 um range of the (circular) guided mode in the optical fiber (usually single-mode). The OMT is the region in an electro-optical device, including the optical device monolithically defined within the electro-optical device, where the shape of the guided wave is transformed. The transformation is obtained by the variation of the shape of the waveguides, such as by tapering. However, carrier-induced change in the refractive index of the material used for the OMT due to doping has increased coupling loss more than expected. The presence of dopants of opposed conductivities, is necessary for the electron transport of active optical devices. In a typical laser, for example, an active region is sandwiched between an n-doped buffer layer and a p-doped over-growth layer. It is known that n-doping changes the refractive index more than p-doping. Although other III-V compounds can be used. InP is often used for high speed optical electronics such as above 10 GHz because an InP device is faster (carrier mobility is higher) than GaAs device. When the InP layer is doped (n or p), free carriers are present in the InP material in the conduction band or in the valence band. These carriers can absorb light (i.e. absorb a photon by a band transition). When the absorption is modified, the index is also modified. Due to the effective mass difference between an electron and a hole, for the same doping level, the index change is more pronounced for n (electron) doping. Therefore, a need exists to reduce this difference in the carrier-induced change in the refractive index of an OMT to further reduce the coupling loss of the OMT.
One aspect of the present invention is a mode transformer that includes a passive waveguide having a first composition sandwiched between two layers of a second composition. A p-doped re-growth layer having the second composition is disposed on top of the passive waveguide. A compensated n-doped buffer is disposed underneath the passive waveguide. The compensated n-doped buffer has the first composition and a sufficient concentration of a third composition such that the compensated n-doped buffer layer has a reduced index difference between the p-doped re-growth layer and the compensated n-doped buffer layer to compensate the index difference between the p-doped re-growth layer and an un-compensated n-doped buffer in order to preserve the symmetry of the guided optical wave.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.